Acetic acid recovery



April 2, 1935. I R w KQSTER 1,996,706

cETIc 011) RECOVERY Filed Nov. 22, 1955 CELLULOSE ACETYLATION MIXTURE ACID DOPE AQUEOUS PREcIPITATIoNbF CELLULOSE ACETATE A. 8 9 I I CELLULGSE ,/7 A AQUEOUS ACETATE ACETIC ACID l3 /l7' Io SODIUM 4 MIXING ACETATE 20 I EQUIPMENT F 30 I 3I' l I v SODIUM CHLORIDE I ANHYDRIDE SIZILL RESIDUE 34 |SOLVENT WAsT PRODUCT 33 I5 l6 I EXTRACTING I8--' 35 F Y EQUIPMENT -"-1 I FORMERLY GOING I TO SEWER JR HEAVY I INE SHOWS! TO I E USE'OF SEWER #23 22 ,I l"* i 'I J A DISTILLATION as EQUIPMENT '26 coNcENTR TED ,7 ACETIC ACID SUBSTANTIALLY ANHYDROUS OR GLACIAL 1|)NVENTOR FREDERICK w. KOSTER I BY HIS ATTORNEYS' Patented Apr; 2,1935- UNITED STATES PATENT OFFICE. Y

1.99am 7 scene Acm ancovany A Frederick'w. Koster, m Pa... "am;-

The Viscose Company, Marcus corporation 0'! Pennsylvania Application November 22, 193:, SerialNo. 699,251

10 Claims- '(0!. zen-12s) This invention relates to the recovery of concentrated acetic acid from dilute aqueous solutions thereof, for example the-weak acetic acid obtained as, a by-product in the manuifacture oi 5 cellulose acetate, and more particularly to methods 01' this character which comprise extracting the acid by means of an organic solvent which is practically immiscible with the aqueous solution, separating the solvent-acid extract from the aqueous extracted solution by decantation, and finally separating the solvent from the acid by distillation. It is the main object of this invention to improve the efliciency and the econ- I omy of the extraction step, along with other ob- 15 jects which may hereinafter appear. One of the chief problems in this process is the difilculty of selecting a proper solvent, particularly when the production of an anhydrous acid is desired. Most of the more volatile organic solvents that have a high distribution ratio for the acid as between solvent and aqueous solution, have the undesirable property of also extracting from the dilute acid solution, av prohibitive amount of water. i

As this water must be subsequently removed it anhydrous acid is to be obtained, the use of such solvents as ethyl ether and ethyl acetate-is uneconomical, although otherwise highly desirable as to partition coeflicients vof distribution. The

acidresulting from ethyl ether extraction is never 'higher than 75%, while ethyl acetate is similar, and has the further disadvantage that with higher concentrations of acetic acid such'as 35% and 40%, it is 'miscible and therefore cannot. be decanted from them.

It has been proposed to treat the solvent-acid I extract with deliquescent salts which will absorb water from the solution and thus leave a more concentrated acid in the solvent. This method is expensive and not practical when dealing with.

large quantities of low priced acids- It has also beenproposedto employ a mixture of extracting solvents to reduce the quantity of water which is extracted with the acid. Volatile hydrocarbons like benzene or petroleum ether, and chlorinated hydrocarbons and other. compounds, .all yield a highly concentrated acid, but their coeflicients' of distribution are very poor. If these are mixed with [solvents having high distribution coefficients, the quan-- MW of water extracted by the mixed solvent is reduced; but at the same time the distribution coefficient is likewise reduced. Hence the efiiciency' of the solvent mixture as an extracting medium is lowered accordingly. With 'tion with the salts.

' ly in the extraction 'tate is precipitated by contact this solvent mixture proposalthereiore, it is necessaiytobalancethe advantages gainedbyobtaining a concentrated acid from the extraction, against the cost oil-removing the extra volume of solvent required by the lowered distribution co- 5 eiiicient and consequent lowered extraction efliciency.

- A further proposal is the addition to themeous acetic acid solution, or salts which are insoluble in the extracting solvent. Thus the dis- 1 tribution coefficient is increwed, in amount depending on the degree of. saturation oi the solu- Hence to obtain exhaustion of the acid in the aqueous solution by the extraction process, smaller volumes-oi solvent are re- 5 quired, and smaller quantities of water are extracted with the acid. It is possible in this mannor to use solvent's oi very high extraction eiliciencies', for example ethyl ether or ethyl acetate, for the production of highly concentrated or even anhydrous acetic acid. -However, the cost of such salts in amount required for producing even partial saturation of the weak acetic acid is pro-' hibitive, when the low market value 01' glacial acetic acid is considered, and is the chief obstacle to the industrial application 01 this'proposal.

. It has been discovered, however, that the advantages of this third proposal may be obtained without the prohibitive cost, by employing in or waste salts such as sodium sulphate and sodium chloride. In the manufacture of acetic anhydride, one o; the operations associated with the production or c'ellulose'acetate, there isobtained as waste material a mixture of sodium chloride and sodium sulphate which is* especially adapted to carrying out the above suggested method. These salts are anhydrous, and usually contain appreciable amounts of acetic acid values which will be recovered automatical- 40 operation.

The drawing is a flow sheet or. the manu1ac-, ture of cellulose acetate as improved by the pres ent invention.

The acid dope or cellulose acetylation mixture 2 comprises cellulose acetate in solution in acetic'acid. This material'is passed-by any suitable means 3 such as a pipe or conveyor, to the precipitationstage I, wherein thecellulose acewith water. The products from the precipitation stage l are removed at 5, and the cellulose acetate precipitate I is separated at 8 from the aqueous acetic acid ill by decantation 8. This cellulose acetate -1 is a primary product, which is washed 5 and ground if necessary for further processing,

incidental to the manufacture of cellulose acetate products, such as rayon.

The aqueous acetic acid II is the product which is tobe concentrated, for example, to be returned to the cellulose acetylation mixture 2;

This aqueous aceticacid is divided into two parts or streams l2 and IS. The part I! is treated to recover sodium acetate ll. Acetic'anhydride I! is produced from this sodium acetate I4, and it is returned by II to the cellulose acetylation stage 2. This acetic anhydride production leaves a still residue ii of sodium sul phate and sodium chloride, which was formerly a waste product, to be passed. for example by l8 to the sewer l9. 1

I'he remaining part [3 of the aqueous acetic acid was formerly passed, as shown by the dotted line 20, directly to extracting equipment 2|.

' In th extracting equipment II, the solventfor example ethyl ether, amyl acetate and/or ethyl acetate, the last being preferred, is introduced as at 22-, preferably in counterflow relation. The solvent-acid-extract is decanted as at 23 and passed to distillation equipment 24, where the solvent is distilled from the acid extract, and condensed and returned by 2! to the extraction stage 2 l,'leaving at 2' a residue of concentrated acetic acid2l which, in substantially anhydrous or glacial form, is returned by 28 to the cellulose acetylation stage 2.

According to one embodimentof the present invention, as shown by the heavy lines, the still residue 16 is passed by II to the stream ll of aqueous acetic acid I I, going to the mixing equipment 3|. From this'stagethe mixture of salts I 6 and acid ll pass by 32 to filtering equipment The filtrate passes by 34 to the extraction stage 2|, while the residue is disposed of by 85, which passes this residue to the sewer".

vThe quantity of water in the extract will depend on the degree to which the aqueous acid is saturated with these salts, the amount of wa-.-

ter extracted decreasing with the quantity ofsalts dissolved. For example, with a 25% acetic acid solution and ethyl ether as an extracting medium, the extracted acid is usually between and in concentration. after removal.

of the ether.

By the addition of 15% of waste salts to the 25% acid solution, the strength of the ether free extracted acid can be raised to Further increases in the salts addition 'result in even stronger acids.

In the case of ethyl acetate, which has one of the highest distribution coefllcients for acetic acid as betweensolvent and water, it is possible n to obtain practically anhydrous acid by the addi- "tion of 15% of these salts to the dilute acid before extraction. In fact it is only possible by such; additions to use this most desirable solvent by itself, because as pointed out. above, there is no separation between it and aqueous acids above 20% in concentration. Such a small amount of .water is extracted by this procedure with ethyl acetate, that it is all carried over as a minimum boiling point mixture with theethyl acetate, as

' the latter is separated from the acid by distillation. The result is an anhydrous residue of' acid.

: Since the acetic anhydride cannot in practical operation be completely distilled from the a still residues, there was formerly an appreciable loss of this product when the residues 'were discharged to the sewer. By dissolving the residues 75 in'the aqueous acetic acid proposed this an- 1,cca,7oe hydride'is converted to acetic acid and is re covered as such during the extraction. I

An incidental iurtheradvantage of this-invention is the salting out effect produced by the addition of inorganic salts on the dissolved 01'1 ganic 'impurities in the dilute acid, the presence of which might otherwise interfere with the extraction or the ,quality of the acid recovered.

It is to be understood that the invention embraces such embodiments of the broad idea as fall within the scope of the appended claims.

I claim:

1. In the manufacture of cellulose acetate in which dilute aqueous acetic acid is produced and concentrated by extraction, and in which acetic anhydride' is produced from part of said dilute, aqueous acetic acid leaving a residue of waste salts. the improvement which comprises dissolving about "15% waste salts from said residue in said dilute aqueous acetic acid solution before the extraction step.

2. In the process of manufacture of cellulose acetate which comprises acetylating cellulose to form acid dope, precipitating cellulose acetate from said acid dope by contacting the acid dope with water, whereby cellulose acetate and aqueous acetic acid are formed, decanting the'aqueous acetic acid from the cellulose acetate, treating a part of said aqueous acetic acid to recover sodium acetate, producing acetic anhydride from the sodium acetate leaving a residue of waste salts, subjecting the other part of the aqueous acetic acid solution to extraction with a solvent, de-

canting the solvent acid layer and distilling oif the solvent from the-concentrated acetic acid,

and returning the concentrated acetic acid to the acetylation stage, the improvement which' comprises dissolving a substantial quantity of waste salts from said residue in said other part of the aqueous acetic acid solution. 7

3. Process of recovering concentrated acetic acid from a dilute aqueous solution thereof produced in the manufacture of cellulose acetate, which comprises dissolving in the aqueous acetic acidsolution a substantial quantity of waste salts which contain acetic acid values from another part of the cellulose acetate process, and subjectingthe resulting solutionto extraction with asolventlargelyimmiscible with water.

4. Process for the recovery of concentrated I acetic acid from a dilute aqueous solution thereof which comprises adding to the solution at least 15% of waste salts from the man acture of cellulose-acetate and containing ace c acid values,.-

acetic acid is concentrated and the acetic acid values from said waste salts are recovered.

5. Imthe manufacture of cellulose acetate in which dilute acetic acid is produced, and waste salts remain from the production of acetic anhydride from part .of said dilute acetic acid, and anand subjecting the solution to extraction with an 3 organic solvent, decanting the solvent acid layer and distilling the solvent therefrom whereby the acetic acid before the extrac- I .sisting of sodium chloride and sodium sulphate aiidconteiningacetlcacidva'im indiiuteaqueon: acetic acid solution. andextractim concentrated acetic acidtheretromw'itha eoivmtlargeb immiscible with water.- i

'7. irom'wastc sgitsproducedinthe'ot which w ii:

ment whichrccoverin tiie vaiuesinsaidwastesaltsbydbolvin subtential quantity of said 'ealtl iliihc' v icid'extractiontmdiiute aqueous the. brod icti 0t acetic anhydride comprises a mbetantial quancn d acid iron the acetic anhydride in aaid'dilutc aqmoua acetic acid beaumcient to produce acetic acidotuom'l5' to more than 85% concentra wvxos'mm otthc miaturc a waste sits and u'nrecov-' of recovering concentrstcd acctic' I achchtfor acetic acid. said waste 

